Originally Posted by merlinextraligh

Standard road bike, no aerobars, rider in drops, versus, full on TT bike, aerobars, good position, and aero wheels, saves a couple of minutes in a 40k TT.

In Kruezotter, 25 mile TT, 300 watts, 157lb rider:

Road bike in drops, 24.3mph, 1hour 1 minute 43 seconds.

Triathlon bike, 58 minutes, 8 seconds.

I don't know the precise assumptions the calculator makes, but that amount of difference is fairly realistic . Although, I think you could argue that its slightly overestimates the difference.